Ventilator apparatus for use in patient care applications is disclosed in, for example EP0830166. The system disclosed includes a ‘bag in bottle’ ventilation system which consists of a bellows provided in a transparent sealed container. This disclosed “bag in bottle” arrangement is intended to be connected to a patient breathing circuit which can be of several standard conventional designs but must include a carbon dioxide scrubbing component such as a container of “soda-lime” as might commonly be used in anaesthesia breathing systems for example. The bellows (and the rest of the chosen breathing circuit to which it is attached) contains either oxygen or a breathing gas mixture which includes enough oxygen to sustain life—typically 21% oxygen or greater (as normal air comprises 21% oxygen).
The container is provided with an inlet enabling a ventilator device to deliver oxygen into the container in a cyclic fashion as a “driving gas” to compress the bellows, so ventilating the lungs via the breathing circuit to which the bellows are attached. The bellows has a one way valve that can open to permit a small portion of “driving gas” oxygen to enter the interior of the bellows (and breathing circuit to which the bellows are attached) but this only occurs when the bellows has lowered successively during successive inhalation/exhalation cycles to abut the ‘bump stops’ at the base of the container. When this occurs the pressure outside the bellows becomes greater than the pressure within it by an amount sufficient to open the one way valve of the bellows. This allows a small portion of the “driving gas” oxygen pumped from the ventilator to pass into the bellows to automatically replace the oxygen therein, according to the needs of the patient. The arrangement of EP0830166 can be used for delivery of anaesthetic to a patient. An advantage of such an arrangement is that it automatically tops itself up with oxygen to replace oxygen metabolically taken up by the patient from their lungs into their blood, during breathing. No anaesthetic gas that has additionally been added to the breathing circuit (to which the bellows is connected) is permitted to escape from the closed loop system. This is beneficial where expensive anaesthetics are used for example Xenon gas, as in this manner, the only loss of anaesthetic gas or vapour from the breathing system is by patient tissue uptake via their lungs. The design becomes particularly cost-efficient as a delivery system with vapors or gases which are taken up only very slowly by the patient, particularly if they are expensive. Xenon is a prime example of a gas which exemplifies both these conditions, while sevoflurane would be an example of an anaesthetic vapour which fulfils these conditions. As disclosed in EP0830166, alternative valve arrangements may be used to top up the oxygen into the closed loop system. Also the bellows can be replaced with other arrangements such as an inflatable rubber bag or similar.
For a system operating efficiently, it is important that ‘top up’ oxygen is permitted to pass into the system only at the end of inspiration via the valve which can be thought of as an “oxygen substitution valve.” A problem with prior art arrangements is that due to inherent constraints in known one way valves, the valve may open or flutter during the inspiratory phase, allowing oxygen to leak in prematurely. If this flutter and premature leakage is very minor then the effect will be negligible, as any residual requirement to replace oxygen is completed when the bellows or bag empties, by the correct operation of the valve at the end of the inspiration phase.
However, if the valve flutter and oxygen leakage into the breathing circuit is excessive, more oxygen can end up being added than was consumed during the previous breath cycle, resulting in the bellows or bag filling and becoming more distended at each end expiration. In other words excessive oxygen is added over time, in excess of the patient uptake. By adding oxygen at a rate matching uptake (correct operation), the gas mixture in the breathing circuit stays constant or only changes slowly over time—this is advantageous for the physician running the anaesthetic procedure for example. If oxygen is added at a rate exceeding patient uptake, the bellows will eventually completely fill and any excess volume is thereafter vented from the circuit via standard conventional safety valves (not shown) present in all such anaesthesia and similar systems to prevent pressure build-up and lung damage. If the circuit “vents” excess gases in this way, it will still function as a mechanical ventilation circuit but any advantages in terms of gas economy offered by its totally closed recirculating nature are then lost. If the patient is a newborn baby, a situation where oxygen is being added (in gross excess of patient uptake) is risky to the health of the baby as an excessively high oxygen concentration in the breathing gas mixture (that would result) can be harmful, for example causing blindness. Where the patient is physically small (for example for a baby) and the oxygen consumption rate is very low in absolute terms, the margin of “error” with respect to correct function of the oxygen substitution valve (opening at end-inspiration only) is very small. In this situation even a small degree of premature valve “flutter” and oxygen addition during the inspiration phase, rather than at end-inspiration, could very easily lead to more oxygen being added than was consumed during the previous breathing cycle. In such a situation, premature opening of the valve is dangerous and unacceptable because too much oxygen will be added over time to the recirculating part of the system.
FIG. 1 shows an exemplary system that will be likely to suffer from the problem described. A mechanical ventilator 101 pumps oxygen into the chamber 102. An inflatable rubber bag 103 is positioned internally of the chamber and is collapsed and expanded during the breathing cycle. A disc valve 104 (the oxygen substitution valve) normally rests in a closed position but when the pressure differential above and below the valve is sufficient the disc valve is caused to be raised to an open position permitting top up oxygen to be delivered to the system. This should only happen at the end of the inspiratory phase in a correctly operating system. The system loop shown includes an integrated carbon dioxide absorber 106 containing for example, soda lime granules.
However, in actual operation, as the mechanical ventilator 101 pumps oxygen, it takes a moment for the pressure in the chamber 102 to be transmitted to the interior of the inflatable rubber bag 103. Consequently it takes a moment for the gas pressure above the disc valve 104 to equalize with the gas pressure below the disc valve 104. Therefore the valve may open or flutter during the inspiratory phase, allowing oxygen to leak into the upper chamber 105 and pass into the closed system prematurely. A crude solution is to spring load the disc valve 104 in order to require more force to open the valve 104. The mechanical energy required to open such a spring loaded valve at the end of inspiration would be provided by the ventilator not the patient so on the face of it this may appear to be acceptable. However such an arrangement is unsatisfactory as it interferes with operation of the ventilator machine by causing an (abnormal) pressure spike in the recorded pressure at the mechanical ventilator.
FIG. 2 shows a pressure vs. time recorded plot for a ventilator without a spring loaded valve (but with a valve prone to the flutter problem). The plot is for the inspiration phase and measures not the pressure in the lungs but the pressure at the point where the mechanical ventilator attaches to the lower chamber 102 (i.e. the pressure in the lower chamber 102). In this plot the opening pressure of the disc valve is minimal (the disc valve is not spring loaded). The valve opens at the end of inspiration (point A) without requiring a rise in pressure generated by the mechanical ventilator in order to open the valve.
FIG. 3 shows a plot similar to FIG. 3, but this time for a disc valve which has been spring loaded to prevent flutter during the inspiration phase. The valve opens at the end of inspiration (point A) but in this instance requires a rise in pressure generated by the mechanical ventilator in order to open the valve. This rise in required opening pressure can be seen as the step at point A. As a result the pressure/time waveform is abnormal. Although not necessarily harmful to a patient certain advanced mechanical ventilators may interpret this abnormality as for example a patient coughing against the ventilator and activate a warning alarm as a result. Such a step increase in the required opening pressure is also undesirable as the peak pressure apparently being delivered to the lungs from the display appears to be 22 cmH2O when in reality it is less than this.